WooCommerce HPOS

$order = wc_get_order( $order_id );
$value = $order->get_meta( $meta_key );

See https://woocommerce.github.io/code-reference/classes/WC-Data.html#method_get_meta

$order = wc_get_order( $order_id );
$order->update_meta_data( 'custom_key', 'value' );
$order->save();

OR
$order = wc_get_order( $order_id );
$order->update_meta_data( $meta_key_1, $meta_value_1 );
$order->add_meta_data( $meta_key_2, $meta_value_2 );
$order->delete_meta_data( $meta_key_3, $meta_value_3 );
$order->save();